The effects of metmyoglobin (MMb) on the quality of cured fish were studied by adding different concentrations of MMb into the cured grass carp to construct the sample reaction system. Based on the carbonyl compounds, lipid content, fatty acid composition and volatile flavor compounds, the mechanism of MMb promoting lipid oxidation in traditional processing of cured grass carp was studied. As a result, the contents of three carbonyl compounds increased gradually, and the increasing trend was "4-hydroxyhexenal > 4-hydroxynonenal > malondialdehyde" with the increase of MMb concentration. Compared with the control group, the contents of total lipids, triglycerides and phospholipids in the 0.8 mg/g MMb group decreased the most (18.95%, 18.72% and 31.83%, respectively). However, the content of free fatty acids showed a completely opposite trend (increased by 6.74%). The higher the concentration of MMb, the easier the oxidative degradation of polyunsaturated fatty acids (PUFA) in fish samples, especially linoleic acid and arachidonic acid (the relative content decreased by 12.37% and 16.27%, respectively). Six categories and 41 kinds of volatile flavor components were detected, and 13 kinds of odor active compounds such as pentanal were identified. All the odor active compounds except decanal were significantly negatively correlated with the relative content of PUFA. It indicated that MMb-mediated lipid photooxidation may be the main mechanism of traditional cured fish aroma.